This invention relates to an adjust mechanism of a drum brake generally used in vehicles, and more particularly, to water proof of that mechanism.
It is a common practice in a drum brake to provide an adjust mechanism to compensate for an increase of piston stroke in the wheel cylinder or an increase of lever stroke in the parking brake due to wearing of the brake shoes. The adjust mechanism is usually disposed on a piston of the wheel cylinder, on a strut of a parking brake mechanism, or an anchor, etc., and it generally includes an adjust bolt and an adjust nut. Principle of this mechanism resides in offsetting (compensating) the increasing of stroke in any of the braking mechanism, such as a wheel cylinder, parking brake lever, etc., owing to wearing of the brake shoes, taking advantage of a relative movement of the adjust bolt and the adjust nut caused by a relative rotation of the two members.
It is therefore required that a smooth relative rotation (rotational movement) between the adjust bolt and the adjust nut should be guaranteed at least for the whole period of life of the brake shoes. In a vehicle drum brake which is not necessarily free from ingress of water, the adjust bolt and the adjust nut are liable to the ingress of water into the gap between external thread (male screw) and internal thread (female screw) which may produce rust there. Smooth rotation is thereby often prevented, and even an entire incapability of rotation is not rare.
Such a non-smooth relative rotation or an entire cease of relative rotation between the two members in a manual stroke adjust mechanism requires repair or change of the mechanism itself, and in an auto-adjust mechanism causes an imperfect operation or sometimes an entire cease of operation of the mechanism. This is very dangerous for the vehicle running.
For the purpose of avoiding such a disadvantage, it has conventionally been practiced to spread grease between both threads. It is true that the above is effective in preventing water ingress in between the contact surfaces of the two and in preventing the rust production there. In addition to difficulty of spreading grease evenly or equally over the entire surface of the two members, an excessive amount of grease applied between both threads with the hope of sufficient anti-rust effect often makes venting in the tapped hole unsatisfactory. As the adjust bolt is threaded into the adjust nut, air is confined in the tapped hole and is liable to disadvantageously repulse or push out the adjust bolt, because the air pressure is gradually accumulated there due to the existence of the grease. Making the matter worse, the grease smeared there is pushed outside a great deal by the accumulated air pressure, causing a loss of the grease.